diff --git a/src/main/runtime/relay/relay-control-client.ts b/src/main/runtime/relay/relay-control-client.ts
index 139d63e5640..c270ad7e322 100644
--- a/src/main/runtime/relay/relay-control-client.ts
+++ b/src/main/runtime/relay/relay-control-client.ts
@@ -9,7 +9,6 @@ import {
   RelayHostChallengeMessageSchema,
   RelayHostHelloAckMessageSchema,
   RelayPingMessageSchema,
-  RELAY_HOST_CAPABILITY_HEADERS,
   encodeRelayHostHello,
   parseRelayControlMessage,
   type RelayConnectionOpenMessage,
@@ -25,6 +24,7 @@ import {
   RelayControlSilenceWatchdog
 } from './relay-control-silence-watchdog'
 import { closeRelayControlSocket } from './relay-control-socket-close'
+import { createRelayControlSocket } from './relay-control-socket-factory'
 import { controlWebSocketUrl } from './relay-control-url'
 
 type RelayControlState = 'idle' | 'opening' | 'proving' | 'active' | 'draining' | 'closed'
@@ -44,6 +44,9 @@ type RelayControlClientOptions = {
   onClose: (code: number) => void
   createSocket?: (url: string, relayJwt: string) => WebSocket
   connectDeadlineMs?: number
+  // Why: settable apart from connectDeadlineMs so a test can stall the connect
+  // phase alone and prove which of the two bounds fired.
+  handshakeTimeoutMs?: number
   silenceLimitMs?: number
 }
 
@@ -74,11 +77,13 @@ export class RelayControlClient {
     this.createSocket =
       options.createSocket ??
       ((url, token) =>
-        new WebSocket(url, {
-          headers: { authorization: `Bearer ${token}`, ...RELAY_HOST_CAPABILITY_HEADERS },
-          perMessageDeflate: false,
-          maxPayload: 64 * 1024
-        }))
+        createRelayControlSocket(
+          url,
+          token,
+          options.handshakeTimeoutMs ??
+            options.connectDeadlineMs ??
+            RELAY_CONTROL_CONNECT_DEADLINE_MS
+        ))
   }
 
   connect(): Promise<RelayHostHelloAckMessage> {
diff --git a/src/main/runtime/relay/relay-control-connect-transport-bound.test.ts b/src/main/runtime/relay/relay-control-connect-transport-bound.test.ts
new file mode 100644
index 00000000000..7d47c0e09fc
--- /dev/null
+++ b/src/main/runtime/relay/relay-control-connect-transport-bound.test.ts
@@ -0,0 +1,93 @@
+import { createHash } from 'node:crypto'
+import { readFileSync } from 'node:fs'
+import { createServer, type Server, type Socket } from 'node:net'
+import { join } from 'node:path'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import nacl from 'tweetnacl'
+import { isRemoteRuntimeConnectTimeout } from '../../../shared/remote-runtime-connect-bound'
+import { RelayControlClient } from './relay-control-client'
+
+const servers = new Set<Server>()
+const sockets = new Set<Socket>()
+
+afterEach(async () => {
+  for (const socket of sockets) {
+    socket.destroy()
+  }
+  sockets.clear()
+  await Promise.all(
+    [...servers].map(
+      (server) =>
+        new Promise<void>((resolve) => {
+          server.close(() => resolve())
+        })
+    )
+  )
+  servers.clear()
+})
+
+/**
+ * Accepts TCP and never answers the HTTP upgrade, so the socket never opens.
+ * The incumbent relay test stalls the *proving* phase instead, which the
+ * transport bound cannot see — only this shape distinguishes the two bounds.
+ */
+async function listenSilentUpgradeServer(): Promise<string> {
+  const server = createServer((socket) => {
+    sockets.add(socket)
+    socket.once('close', () => sockets.delete(socket))
+  })
+  servers.add(server)
+  await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve))
+  const address = server.address()
+  if (address === null || typeof address === 'string') {
+    throw new Error('expected a TCP address')
+  }
+  return `http://127.0.0.1:${address.port}`
+}
+
+function buildClient(cellUrl: string, overrides: { handshakeTimeoutMs?: number }) {
+  const keypair = nacl.box.keyPair()
+  return new RelayControlClient({
+    cellUrl,
+    relayJwt: 'scoped-token',
+    relayHostId: createHash('sha256').update(keypair.publicKey).digest('base64url').slice(0, 16),
+    assignmentEpoch: 1,
+    identity: { userId: 'user-1', profileId: 'profile-1', organizationId: 'org-1' },
+    keypair: { ...keypair, publicKeyB64: Buffer.from(keypair.publicKey).toString('base64') },
+    appVersion: '1.2.3',
+    onConnectionOpen: vi.fn(),
+    onDrain: vi.fn(),
+    onClose: vi.fn(),
+    // Why: 50x the transport bound, so whichever error arrives names the bound
+    // that produced it rather than the one that merely exists.
+    connectDeadlineMs: 5_000,
+    ...overrides
+  })
+}
+
+describe('relay control connect transport bound', () => {
+  it('bounds a connect that never opens, and names the transport bound', async () => {
+    const cellUrl = await listenSilentUpgradeServer()
+    const client = buildClient(cellUrl, { handshakeTimeoutMs: 100 })
+
+    const error = await client.connect().then(
+      () => null,
+      (reason: unknown) => reason as Error
+    )
+
+    expect(error).toBeInstanceOf(Error)
+    // The transport bound fired, not the class deadline that also covers this.
+    expect(isRemoteRuntimeConnectTimeout(error)).toBe(true)
+    expect(error?.message).not.toContain('relay_control_connect_timeout')
+  })
+
+  // Why: defence in depth only works if both bounds survive. Either one removed
+  // as "redundant" leaves a phase uncovered.
+  it('keeps both bounds, since each covers a phase the other cannot', () => {
+    const client = readFileSync(join(__dirname, 'relay-control-client.ts'), 'utf8')
+    const factory = readFileSync(join(__dirname, 'relay-control-socket-factory.ts'), 'utf8')
+    expect(factory).toContain('remoteRuntimeConnectOptions(')
+    expect(client).toContain('this.connectTimer = setTimeout(')
+    expect(client).toContain("new Error('relay_control_connect_timeout')")
+  })
+})
diff --git a/src/main/runtime/relay/relay-control-socket-factory.ts b/src/main/runtime/relay/relay-control-socket-factory.ts
new file mode 100644
index 00000000000..5f7d3b90d6c
--- /dev/null
+++ b/src/main/runtime/relay/relay-control-socket-factory.ts
@@ -0,0 +1,35 @@
+import WebSocket from 'ws'
+import { remoteRuntimeConnectOptions } from '../../../shared/remote-runtime-connect-bound'
+import { RELAY_HOST_CAPABILITY_HEADERS } from './relay-control-protocol'
+
+/**
+ * Builds the relay control socket with a transport-level connect bound.
+ *
+ * Why this exists separately from `connectDeadlineMs` in `RelayControlClient`:
+ * that deadline bounds the whole handshake including the host proof, but it can
+ * only run once the socket object exists, and it is the class's to arm. A
+ * black-holed relay never opens and never errors, so the connect sub-phase
+ * needs its own bound at the transport — the same one the remote-runtime
+ * transports use. Both are kept deliberately: they cover different phases and
+ * neither is redundant. See #18191.
+ *
+ * Keeping the construction here means a caller that reaches for the relay
+ * control socket gets the bound, rather than re-deriving an unbounded one.
+ */
+export function createRelayControlSocket(
+  url: string,
+  relayJwt: string,
+  connectBoundMs: number
+): WebSocket {
+  return new WebSocket(
+    url,
+    remoteRuntimeConnectOptions(
+      {
+        headers: { authorization: `Bearer ${relayJwt}`, ...RELAY_HOST_CAPABILITY_HEADERS },
+        perMessageDeflate: false,
+        maxPayload: 64 * 1024
+      },
+      connectBoundMs
+    )
+  )
+}
diff --git a/src/shared/remote-runtime-connect-bound.test.ts b/src/shared/remote-runtime-connect-bound.test.ts
index de2af2bc1bb..2d8cc98feef 100644
--- a/src/shared/remote-runtime-connect-bound.test.ts
+++ b/src/shared/remote-runtime-connect-bound.test.ts
@@ -1,6 +1,6 @@
-import { readFileSync, readdirSync } from 'node:fs'
+import { existsSync, readFileSync, readdirSync } from 'node:fs'
 import { createServer, type Server, type Socket } from 'node:net'
-import { join } from 'node:path'
+import { basename, join } from 'node:path'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { generateKeyPair, publicKeyToBase64 } from './e2ee-crypto'
 import type { RemoteRuntimeClientError } from './remote-runtime-client-error'
@@ -15,6 +15,36 @@ import { openRemoteRuntimeWebSocket } from './remote-runtime-request-websocket'
 const servers = new Set<Server>()
 const sockets = new Set<Socket>()
 
+const RELAY_CONTROL_SOCKET_FACTORY = join('relay', 'relay-control-socket-factory.ts')
+
+/**
+ * Files whose WebSocket construction must carry the connect bound. The shared
+ * remote-runtime transports are swept by prefix; sites outside this directory
+ * are listed explicitly so adding one is a deliberate act rather than a glob
+ * accident. Other WebSocket sites (relay data transport, emulator control)
+ * carry their own bounds and are deliberately not covered here.
+ */
+function coveredSocketSources(): string[] {
+  const shared = readdirSync(__dirname)
+    .filter(
+      (name) =>
+        name.startsWith('remote-runtime-') && name.endsWith('.ts') && !name.includes('.test.')
+    )
+    .map((name) => join(__dirname, name))
+  const relaySocketFactory = join(
+    __dirname,
+    '..',
+    'main',
+    'runtime',
+    'relay',
+    'relay-control-socket-factory.ts'
+  )
+  if (!existsSync(relaySocketFactory)) {
+    throw new Error(`connect-bound ratchet lost its relay site: ${relaySocketFactory}`)
+  }
+  return [...shared, relaySocketFactory]
+}
+
 afterEach(async () => {
   for (const socket of sockets) {
     socket.destroy()
@@ -59,25 +89,26 @@ describe('remote runtime connect bound', () => {
 
   // Why: the bound only helps if every Node-side remote-runtime socket carries
   // it; a new transport that calls `new WebSocket` directly reintroduces #18191.
-  it('routes every shared remote-runtime WebSocket through the bounded options', () => {
-    const dir = join(__dirname)
+  it('routes every covered WebSocket construction through the bounded options', () => {
     const offenders: string[] = []
     let scannedConstructions = 0
-    for (const name of readdirSync(dir)) {
-      if (!name.startsWith('remote-runtime-') || !name.endsWith('.ts') || name.includes('.test.')) {
-        continue
-      }
-      const source = readFileSync(join(dir, name), 'utf8')
+    for (const path of coveredSocketSources()) {
+      const source = readFileSync(path, 'utf8')
       const constructions = source.split('new WebSocket(').length - 1
       const bounded = source.split('remoteRuntimeConnectOptions(').length - 1
       scannedConstructions += constructions
       if (constructions > bounded) {
-        offenders.push(`${name}: ${constructions} WebSocket(s), ${bounded} bounded`)
+        offenders.push(`${basename(path)}: ${constructions} WebSocket(s), ${bounded} bounded`)
       }
     }
     expect(offenders).toEqual([])
     // Guards against the scan silently matching nothing and passing vacuously.
     expect(scannedConstructions).toBeGreaterThan(0)
+    // Guards the relay site specifically: an allowlist that quietly stopped
+    // resolving a path would still satisfy the count above.
+    expect(
+      coveredSocketSources().some((path) => path.endsWith(RELAY_CONTROL_SOCKET_FACTORY))
+    ).toBe(true)
   })
 
   it('reports an unanswered host as unreachable rather than as an empty result', async () => {